The present invention relates to modified feedback control techniques and more particularly to a manipulation-variable separated or a divided type control method and apparatus which are applicable to the continuous and/or discrete control of process quantities such as flow rate, temperature, electric current and the like.
Typically, a feedback control system as shown in FIG. 1 is incorporated in a prior art control system for use in the control of flow rate and temperature.
In such a control system, as well known in the art, the following relations are established between a reference value S of the controlled variable applied to a set point, a feedback variable D, a deviation .epsilon. and a manipulation variable C.sub.o for an object or system 13 being controlled EQU .epsilon.=S-D (1) EQU C.sub.o =G(S).epsilon. (2)
where G(S) is a transfer function of a control element 12.
Thus, in the control system, the deviation .epsilon. between reference value S and feedback variable D is determined at a summing point 11 and processed at the control element 12 by subjecting it to a proportional (P) operation, proportional plus integral (PI) operation or proportional plus integral plus derivative (PID) operation so as to be applied, in the form of a manipulation variable, to the object or system being controlled (hereinafter referred to as the controlled object), whereby the control operation is carried out to make the deviation .epsilon. zero. The controlled variable is detected in terms of an absolute value by an absolute-value detector 16 connected to a point 17 of the controlled object 13. A signal of the detected absolute-value is subjected to a given transformation at a feedback circuit 14 to produce a feedback signal D. As will be described later, between the control element 12 and the controlled object 13 is connected a change rate limiter 15' which confines, as desired, the change rate of a manipulation variable within a fixed range. The limiter 15' normally acts to limit the output of the control element 12 and produces an output on a line 15.
Of various configurations of existing control systems, the construction shown in FIG. 1 is typical and popular for the control of analog quantities.
With the feedback control system of FIG. 1, if the feedback loop is placed in a disconnected state for some reason, .epsilon.=S results from D=0 as will be seen from equation (1) and the instant the feedback loop is opened, a great disturbance is exerted on the control system.
Accordingly, when the object being controlled has a quick response and hence the exertion of such a disturbance is not allowable, it is general practice to set up the control system redundantly or to provide the change rate limiter 15' for the purposes of making a fail safe system. However, the redundant control system needs a complicated control apparatus and is expensive. The provision of the change rate limiter impairs the response of the control system and is liable to failure to attain the intended control. Meanwhile, the recent widespread use of digital computers in process control is remarkable. In this type of process control, a sampling control system is employed whose control scheme per se has a preference for incorporation of the feedback control system as shown in FIG. 1. However, even in an instance where the adder 11, control element 12 and feedback circuit 14 are digitized, the above problems inherent to the feedback system, that is, the problems concerning reliability, stabilization and simplification of the system still remain unsolved.